Electromechanical transducers are machines, which convert electrical energy into mechanical energy or vice versa. An electric motor is a widely used electromechanical transducer that converts electrical energy into mechanical energy using magnetic field linkage. An electric generator is an electromechanical transducer that converts mechanical energy into electrical energy also using a magnetic field linkage.
An electromechanical transducer comprises a stator and a rotor. The stator is an assembly, which represents the stationary part of an electromechanical transducer. The rotor is an assembly, which represents the moving part of an electromechanical transducer. The above mentioned magnetic field linkage couples the stator with the rotor.
In recent years, especially since the introduction of rare-earth magnetic materials, permanent magnet (PM) electromechanical transducers have become popular since they eliminate the need for commutators and brushes, which are commonly used with conventional Direct Current electromechanical transducer. The absence of an external electrical rotor excitation eliminates losses on the rotor and makes PM electromechanical transducers more efficient. Further, the brushless design of a PM electromechanical transducer allows conductor coils to be located exclusively in the stationary stator. In this respect it is mentioned that non-PM electromechanical transducers, which are equipped with commutators and brushes, are susceptible to significantly higher maintenance costs.
PM electromechanical transducers are also known for their durability, controllability, and absence of electrical sparking. Thanks to their advantages PM electromechanical transducers are widely used in many applications such as electric vehicles (i.e. the electromechanical transducer is a motor) or in power generation systems (i.e. the electromechanical transducer is a generator) such as for instance in wind turbines. PM electromechanical transducers are used in particular in so called Direct Drive (DD) wind turbines, wherein the generator is directly connected via a main shaft to (a hub of) a rotor of the respective DD wind turbine.
One known technical problem of PM electromechanical transducers is thermal load acting on the respective permanent magnet material. Specifically, waste heat being generated by stator coils of a stator of a PM electromechanical transducer can cause the PM material to be heated up such that there is a serious risk of a demagnetization of the PM material. Such a demagnetization would dramatically decrease the performance of the respective electromechanical transducer.
It is known to use an air gap between the stator and the rotor of an electromechanical transducer as a cooling duct, through which a cooling fluid is guided in order to remove the waste heat generated by the stator coils. The cooling fluid, which may be e.g. air, can be driven along the air gap by means of a natural convection through the PM electromechanical transducer.